CSS Line Grid Module

Abstract

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This module contains the features of CSS level 3 relating to DESCRIBE HERE. It includes and extends the functionality of CSS level 2 [[CSS21]], which builds on CSS level 1 [[CSS1]]. The main extensions compared to level 2 are SUMMARIZE HERE.

Status of this document

The following features are at risk: …

Table of contents

Introduction

Background

This section is not normative.

This specification provides features to align lines and blocks to invisible grids in the document.

Aligning lines and blocks to grids provides the following benefits:

• Vertical rhythm is kept for better readability.
• Lines are aligned between columns in multi-column documents.
• The top and the bottom margins of pictures are made equal, while keeping the vertical rhythm of text before and after the pictures.
• Layout lines are at the same position on every page in paged media. Keeping the position of the bottom line of a page has benefits for design and readability. This also improves the readability of duplex printing, two pages spreads, and displaying on slow display devices like e-ink.
• East Asian layouts require vertical rhythm more often than other scripts do, even in single column, non-paged media documents, as defined in [[JLREQ]].

There are several types of objects in a document that can break the vertical rhythm. Examples include lines with different sizes of text, pictures, and tables.

Vertical rhythm kept through pictures and different size of text in a multi-column document.

East Asian layouts may require width be a multiple of em without fractions.

East Asian layouts may require grid-like features in inline progression direction as well.

It is often desirable in East Asian layouts to make the line width a multiple of em without fractions. Because most East Asian characters have 1em advance and most East Asian documents are justified, this minimizes cases where justification needs to expand character spacing.

This module provides the following capabilities:

• Defining grids in the line progression direction.
• Controling how lines and blocks align to the grids.
• Rounding lengths to multiples of units.

It is important to control these capabilities independently, so that, for example, aligning to grids can be turned off for tables, but can then be turned back on for aligning the following text to the grids.

Module Interactions

Explain, normatively, how this module affects the definition of CSS.

This module replaces and extends the SUMMARIZE HERE features defined in [[!CSS21]] sections W.X and Y.Z.

Document Conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [[!RFC2119]]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Values

This specification follows the CSS property definition conventions from [[!CSS21]]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS modules may expand the definitions of these value types: for example [[CSS3COLOR]], when combined with this module, expands the definition of the <color> value type as used in this specification.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

Defining Grids

Define Line Grids: the 'layout-grid-line' property

This property defines a line grid for the element and its descendent elements. Possible values are:

An implementer said the grid's scope should be limited to the same BFC, as referring property values and position of parent elements could be difficult to implement if the element is in different BFC than the parent. This can mean a float cannot share the grids with its parent, which is not ideal. How can we solve this?

''none''

No line grid is specified.

''auto''

The grid line size is determined by the computed value of the 'line-height' property.

<length>

Size of the line grid's unit space (a.k.a. "line pitch").

<percent>

Size of the line grid's unit space relative to the logical height of the parent element. If the parent has no explicit height, it is treated as ''auto''. This was defined in CSS3 module: text WD 20010517, but not sure how much useful this is

The defined grid uses the position of the element as the origin of the grid. It uses the top of the element as the origin of the grid for horiozntal text flow. Similarly, it uses the right of the element as the origin of the grid for 'writing-mode' is ''vertical-rl'', and the left of the element for ''vertical-lr''.

Define behavior for paged media/column break

TBD: I might need to copy more text from CSS3 module: text WD 20010517.

Using Grids

Grid Mode: the 'layout-grid-mode' property

This property controls how lines and blocks are aligned to grids. Values have the following meanings:

''none''

Do not align any objects to the grids.

''baseline''

Align baselines to the line grids. The height is the height of the line box. Increasing heights to fit to grids is done by increasing the computed height of the line box. How to make this value work well with images, headings, etc. is still an issue

''block''

Align blocks to the line grids. The height includes paddings and borders, but not margins. Increasing heights to fit to grids is done by adding to computed margins.

''line''

Align line boxes to the line grids. The height is the height of the line box. Increasing heights to fit to grids is done by increasing the computed height of the line box.

Handling margins and borders needs review

When this property is set to anything other than ''none'', the computed height of the object is increased to the smallest multiple of the grid height that is equal to or smaller than the original computed height.

When the value of this property is ''baseline'', the line box is then moved so that its baseline is aligned to the next closest grid.

When the value of this property is either ''block'' or ''line'', the object is vertically centered within the grid row. If the computed height is larger than the grid space, then the object is centered within the smallest number of grid rows necessary for the object to fit in. The starting alignment point is moved accordingly. This is illustrated below, where a represents the numerical 'layout-grid-line' value, and b and c are half the difference between the computed line-heights and either the layout-grid-line value or twice the value respectively.

Layout of content within a line grid, where a represents the layout-grid-line value, and the alignment baseline is at the bottom (after-edge).

As for regular height computation, the ruby box is treated specifically. In a similar fashion to the line-height determination process, only the ruby base element is considered for centering purposes, however the above and/or the after space should be sufficient to fit the ruby text above and/or after the base text within the grid-line space. If this is not the case, the grid-line size needs to be increased in multiple grid rows until the whole ruby box fits. Is just ignoring ruby text enough? like rt { layout-grid-mode: ignore; } or nothing needed to be defined if line-stacking-ruby: auto does the work?

Note that in order for this property to have an effect, line grids must be defined by the 'layout-grid-line' property.

It is probably more desirable for Roman typography to align baselines to grids rather than center of line boxes if we consider cases like only a word in a line is large. But how can we align baselines to grids? Align baselines to center of grids? Lines may not fit within one grid. Align baselines to grids might work better, but it may not work well with settings headings to ''block''.

Need to think more about use cases in tables. Turning off line grids for table cells is one easy way and probably is good enough when all cells are single line, but there probably are more use cases.

Need to think more about use cases in forms.

Large text wraps within line grids.

When a different size of text, such as a headings, wraps, it is usually aligned to grids as a block and the lines within the block do not align.

This effect can be achieved by the following code:

body { line-grid-mode: line; }
h1 { line-grid-mode: block; }

Grid Shorthand: the 'layout-grid' property

TBD

The ''gd'' length unit

The existence of a grid in an element makes it possible and very useful to express various measurements in that element in terms of grid units. Grid units are used very frequently in East Asian typography, especially for the left, right, top and bottom element margins.

Therefore, a new length unit is necessary, gd, to enable the author to specify the various measurements in terms of the grid.

For example, consider the following style:

P { layout-grid: strict both 20pt 15pt; margin: 1gd 3gd 1gd 2gd }

This way, all P elements would effectively acquire a 15pt top margin, a 60pt right margin, a 15pt bottom margin and a 40pt left margin.

If no grid is specified, the gd unit should be treated the same as the em unit.

Just copied from old WD, need to refresh

p {
  layout-grid-line: 1.5;
  layout-grid-mode: baseline;
  margin-top: 1gd;
  margin-bottom: 1gd;
}

h1 {
  line-grid-mode: block;
  min-height: 3gd; /* 3 Gyou-dori */
  margin-top: 1gd;
}

''gem'' length unit is also requested

Forcing Line Width to Multiple of em

There are 3 ideas for this, and all ideas are listed here for disucssions.

Character Grid: the 'layout-grid-char' property

This property defines a character grid for the element. Possible values are:

When a character grid is defined, start and end edges of a line box snap to the nearest grid in the direction to shorten the line box.

Should we add ''edges'' to the 'line-grid-mode' property?

Rounding Lengths to Multiple of Units: ''rounddown()'' and ''roundup()''

The 'width' and 'height' properties accept ''rounddown(length)'' and ''roundup(length)'' values.

The ''rounddown(length)'' computes to ''auto'', and then the result will be rounded down to the largest multiple of the specified unit that is equal to or less than the original computed value.

Similarly, the ''roundup(length)'' computes to ''auto'', and then the result will be rounded up to the smallest multiple of the specified unit that is equal to or greater than the original computed value.

The fraction is distributed evenly to the computed values of margins on each side. If it is used for 'height', half the fraction is added to the top and the bottom margins. If for 'width', to the left and the right margins.

When the element is a float, the fraction is added to the opposite side of the float direction instead. For example, if the 'float' property is ''left'', the fraction is added to the right margins.

Is this design appropriate to solve text with floats case?

Are there requirements for table cell width?

Should have an option to left/right align instead of center? Can author align running head and page numbers with body with this design?

Use cases for non-East Asian layouts?

body {
  margin: 3cm 3.5cm 3cm 3cm;
  text-align: justify;
  width: rounddown(1em);
}
.sidefigure {
  float: right;
  width: roundup(1em);
}

Rounding Lengths to Multiple of Units: the 'layout-grid-measure' property

Conformance

Conformance Classes

Conformance to CSS TEMPLATE Module is defined for three conformance classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to CSS TEMPLATE Module if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.

A renderer is conformant to CSS TEMPLATE Module if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the properties defined by CSS TEMPLATE Module by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to CSS TEMPLATE Module if it writes syntactically correct style sheets, according to the generic CSS grammar and the individual grammars of each property in this module.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental Implementations

To avoid clashes with future CSS features, the CSS specifications reserve a prefixed syntax for proprietary property and value extensions to CSS. The CSS Working Group recommends that experimental implementations of features in CSS Working Drafts also use vendor-prefixed property or value names. This avoids any incompatibilities with future changes in the draft. Once a specification reaches the Candidate Recommendation stage, implementors should implement the non-prefixed syntax for any feature they consider to be correctly implemented according to spec.

CR Exit Criteria

[Change or remove the following CR exit criteria if the spec is not a module, but, e.g., a Note or a profile. This text was decided on 2008-06-04.]

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

Acknowledgments

[acknowledgments]

References

Normative references

Other references

Index

Property index